avcodec/mv30: Do not allow MVs outside the allocated image

Fixes: out of array read
Fixes: 21804/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MV30_fuzzer-5673678898724864

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mv30.c

@@ -545,8 +545,8 @@ static int decode_inter(AVCodecContext *avctx, GetBitContext *gb,
                 int px = x + mv_x;
                 int py = y + mv_y;
 
-                if (px < 0 || px >= avctx->width ||
-                    py < 0 || py >= avctx->height)
+                if (px < 0 || px > FFALIGN(avctx->width , 16) - 16 ||
+                    py < 0 || py > FFALIGN(avctx->height, 16) - 16)
                     return AVERROR_INVALIDDATA;
 
                 src[0] = prev->data[0] + in_linesize[0] * py + px;